Solid memory

ABSTRACT

In one embodiment of the present invention, recording and erasing of data in PRAM have hitherto been performed based on a change in physical characteristics caused by primary phase-transformation of a crystalline state and an amorphous state of a chalcogen compound including Te which serves as a recording material. Since, however, a recording thin film is formed of a polycrystal but not a single crystal, a variation in resistance values occurs and a change in volume caused upon phase-transition has placed a limit on the number of times of readout of the record. The above problem is solved by preparing a solid memory having a superlattice structure with a thin film containing Sb and a thin film containing Te. The solid memory can realize the number of times of repeated recording and erasing of 10 15 .

TECHNICAL FIELD

The present invention relates to a phase-separation solid memory forrecording and erasing, as data, a difference in electric resistance oroptical characteristics which is caused by phase-separation (spindleseparation) of a chalcogen compound which is a form of phase-change.Because phase-separation is a form of phase-change, the phase-separationsolid memory also can be described as a phase-change solid memory(phase-separation RAM, PRAM).

BACKGROUND ART

Recording and erasing of data in phase-change RAM have hitherto beenperformed based on a change in physical characteristics caused byprimary phase-transformation between a crystalline state and anamorphous state of a chalcogen compound including Te which serves as arecording material, and phase-change RAM has been designed based on thisbasic principle (for example, see Patent Literature 1 below).

A Recording material used for recording and erasing data in aphase-change RAM is generally formed between electrodes by using avacuum film formation method such as sputtering. Usually, asingle-layered alloy thin film made by using a target made of a compoundis used as such recording material.

Therefore, a recording thin film of 20-50 nm in thickness consists of apolycrystal but not a single crystal.

A difference in interfacial electric resistance between individualmicrocrystals influences uniformity in electric resistance valuesthroughout a phase-change RAM as a whole, and causes variations inresistance values in a crystalline state (see Non Patent Literature 1below).

Furthermore, it is considered that about 10% change in volume generatedin phase-transition between a crystalline state and an amorphous statecauses individual microcrystals to have different stresses, and flow ofmaterial and deformation of an entire film restrict the number of timesof readout of record (see Non Patent Literature 2 below).

Patent Literature 1: Japanese Patent Application Publication, Tokukai,No. 2002-203392 A

Non Patent Literature 1: supervisor: Masahiro Okuda, Zisedai HikariKiroku Gizyutsu to Zairyo (Technology and Materials for Future OpticalMemories), CMC Publishing Company, issued on Jan. 31, 2004, p 114Non Patent Literature 2: supervisor: Yoshito Kadota, Hikari Disc Storageno Kiso to Oyo, edited by The Institute of Electronics, Information andCommunication Engineer (IEICE), third impression of the first editionissued on Jun. 1, 2001, p 209

Non Patent Literature 3: Y. Yamanda & T. Matsunaga, Journal of AppliedPhysics, 88, (2000) p 7020-7028

Non Patent Literature 4: A. Kolobov et al. Nature Materials 3 (2004) p703

SUMMARY OF INVENTION Technical Problem

Regarding a crystalline structure and an amorphous structure of achalcogen compound including Te, the structural analysis has been madeby X-ray and so on since the latter 1980s. However, since the atomicnumber of Te is next to that of Sb atoms which form the compound with Teand the number of electrons of Te is different from that of Sb atomsonly by one, X-ray diffraction and electron ray diffraction have hardlysucceeded in discriminating Te from Sb. Consequently, detail of thecrystalline structure of the chalcogen compound had been unclear until2004.

Particularly, experiments have demonstrated that characteristics of acompound called GeSbTe (225 composition) and compositions prepared basedon a pseudobinary compound (a compound prepared based on GeTe—Sb₂Te₃,i.e. 225, 147 and 125 compositions), which have been alreadycommercialized in the field of rewritable optical discs, are veryexcellent. However, it has been considered that crystalline structuresof the compound and the compositions are sodium chloride structures withTe occupying a site (site (a)) which Na occupies and with Ge or Sboccupying a site (site (b)) which Cl occupies, and the way of occupyingis random (see Non Patent Literature 3 above).

Solution to Problem

When structural analysis of a GeSbTe compound was made minutely by asynchrotron radiation orbit unit and so on, it was found that achalcogen compound including Te took on a different aspect from aconventional structure in the following points (see Non PatentLiterature 4 above).

1. In a crystalline phase, orderings of Ge atoms and Sb atoms whichoccupy positions of Cl (site (b)) within NaCl—simple cubic lattices arenot in a “random” state as having been considered so far, but positionsof orderings of atoms are properly “determined”. Furthermore, latticesare twisted (see FIG. 1).

2. In an amorphous state, orderings of atoms are not entirely random,but Ge atoms within crystalline lattices are positioned closer to Teatoms by 2 A from the center (though a bit misaligned andferroelectric), and the amorphous state has a twisted structure whilemaintaining its atom unit (see FIG. 2).

3. Restoration of the twisted unit enables high-speed switching to berepeated stably (see FIG. 3).

However, rewritable optical discs without Ge are also commercialized. InDVD-RW and DVD+RW, materials consisting mainly of Sb and additionallycontaining Te are used, and particularly, composition including Sb₂Te ismainly used.

A model of the GeSbTe alloy including Ge was applied to an alloyincluding Sb and Te, and the result of the application was minutelyanalyzed by experiments and computer simulation. As a result, it wasfound that in a chalcogen compound including Ge, a recording or erasingstate is formed by changing positions of Ge atoms as shown in FIG. 1 orFIG. 2, whereas in the alloy including Sb and Te, a large amount ofchange in optical characteristics and in electric resistance are causedby a little interlaminar separation between a Sb₂Te₃ layer and a Sblayer.

From a principle of the interlaminar separation switching which wasnewly found, it was found that formation of a chalcogen compound withoutGe by the following method allows providing a new phase-separation RAMcapable of reducing interfacial electric resistance between individualmicrocrystals as much as possible, and of drastically increasing thenumber of times of repeated rewriting.

That is, it was found that a new phase-separation RAM which drasticallyimproves characteristics of a conventional phase-change RAM can beprovided by artificially forming a chalcogen compound including Sb andTe as a superlattice including a Sb thin film and a Sb₂Te₃ thin film,combining a Sb layer with a Sb₂Te₃ layer via a weak atomic bond, cuttingthe combination only in an interlaminar direction by electric energy andforming and fixing a state with high electric resistance (a state ofrecording (erasing)), and recombining by electric energy and restoring astate with low electric resistance (a state of erasing (recording)).

FIG. 4 illustrates a basic structure of this arrangement. For example,in a case of Sb₂Te, the thickness of a Sb layer is about 0.9 nm, and thethickness of a Sb₂Te₃ layer is about 0.8 nm. Generally, the thickness ofeach layer is preferably from 0.3 to 2 nm.

In a case of forming such an artificial superlattice by sputtering, itis preferable that a speed of film formation per time with respect to anelectric power required for sputtering be measured in advance by using acompound target including Sb or Sb₂Te₃ (or by using single target). Bydoing this, only controlling a time for the film formation allows easilyforming such an artificial superlattice structure including these films.

In a case of forming a single-layered recording film with use of acompound target including composition of Sb and Te, a direction ofinterlaminar separation within a resulting microcrystal is random withrespect to each microcrystal, and electric energy given in order to cutinterlaminar combination does not have coherency, hence a lot of heatenergy has to be wasted as entropy to a system thermodynamically,whereas in a superlattice structure of the present invention, switchingmotion by interlaminar combination is performed in a single direction(that is, having coherency) in a recording film as shown in FIG. 4,plentiful input energy is available for energy as a work, and amount ofenergy wasted as heat (entropy) can be reduced.

Therefore, energy efficiency for performing switching motion byinterlaminar combination is improved. Furthermore, limiting a change involume (change in volume between a crystalline state and an amorphousstate) caused by rewriting only to a uniaxial direction (that is, awork) between layers allows operation of stably repeated rewritingwithout composition segregation.

ADVANTAGEOUS EFFECTS OF INVENTION

With the present invention, formation of a superlattice structureincluding made of a chalcogen compound with different compositionswithout Ge enables characteristics of a phase-change RAM having achalcogen compound including Ge to be improved drastically.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a crystalline structure of Ge—Sb—Te alloy. White circlerepresents Te, black triangle represents Ge and black circle representsSb.

FIG. 2 shows an amorphous structure (short-distance structure) ofGe—Sb—Te alloy.

FIG. 3 shows a basic cell for switching of a phase-change RAM.

FIG. 4 shows a superlattice structure (a combined state) including Sband Sb₂Te₃.

FIG. 5 shows a superlattice structure (a separated state) including Sband Sb₂Te₃.

BEST MODE FOR CARRYING OUT THE INVENTION

Best mode for carrying out the present invention is described below.

Example 1

A phase-separation RAM was formed using a basic technique of generalself-resistance heating. That is, TiN was used for an electrode. 20layers of superlattices of Sb and Sb₂Te₃ were laminated and the laminatewas used as a recording film. The size of a cell is 100×100 nm² square.

Comparison between FIG. 4 and FIG. 5 shows that, in FIG. 5, an interfacebetween Sb atoms and Te atoms below the Sb atoms is a bit broader thanthat in FIG. 4. Such a little difference makes a great difference inelectric conductivity.

A voltage was applied on this device programmatically and current valuesin recording and erasing were measured. The results of the measurementsshow that in recording, the current value was 0.35 mA and the time ofpulse was 5 ns, and in erasing, the current value was 0.08 mA and thetime of pulse was 60 ns. The number of times of repeated recording anderasing at these current values was measured to be 10¹⁴.

Reference Example

A phase-change RAM was formed using a technique of generalself-resistance heating as in Example 1. A 20 nm single-layered film ofSb₂Te was formed for a recording film. The size of a cell was 100×100nm² square. A voltage was applied on this device programmatically andcurrent values in recording and erasing were measured.

As a result, the current value in recording was 1.3 mA and the currentvalue in erasing was 0.65 mA. Note that irradiation time of pulse wasthe same as in Example 1. The number of times of repeated recording anderasing at these current values was measured to be 10¹¹.

INDUSTRIAL APPLICABILITY

In the present invention, the current value in recording data in aphase-change RAM can be decreased to be one-tenth or less, and thenumber of times of repeated rewriting of data can be increased by 2-3digits or more, compared with a conventional phase-change RAM.Therefore, the present invention can make meaningful contribution to theindustry.

1. A Solid Memory, electric characteristics thereof changing due tophase-separation of a substance constituting the solid memory, thesubstance serving as a material for recording and reproducing data, thematerial including a laminated structure of artificial superlatticeswhose electric characteristics change due to the phase-separation. 2.The solid memory as set forth in claim 1, wherein: the laminatedstructure is made of alloy thin films including stibium (Sb) atoms andalloy thin films including tellurium (Te) atoms.
 3. The solid memory asset forth in claim 1, wherein: a thickness of each of the alloy thinfilms including stibium (Sb) atoms and the alloy thin films includingtellurium (Te) atoms ranges from 0.3 to 2 nm.
 4. The solid memory as setforth in claim 2, wherein: data is recorded by causing interfacesbetween the alloy thin films including stibium (Sb) atoms and the alloythin films including tellurium (Te) atoms to be in a one-dimensionallyanisotropically separated state.
 5. The solid memory as set forth inclaim 2, wherein: data is erased by causing interfaces between the alloythin films including stibium (Sb) atoms and the alloy thin filmsincluding tellurium (Te) atoms, having been in a one-dimensionallyanisotropically separated state, to be in a recombined state.
 6. Thesolid memory as set forth in claim 2, wherein: a thickness of each ofthe alloy thin films including stibium (Sb) atoms and the alloy thinfilms including tellurium (Te) atoms ranges from 0.3 to 2 nm.